A semiconductor device manufacturing process uses various thermal processing systems for processing workpieces, such as semiconductor wafers, by thermal processes including an oxidation process, a diffusion process and a film formation process. For example, a known batch vertical thermal processing apparatus is capable of processing a plurality of workpieces simultaneously by a thermal process.
This known vertical thermal processing apparatus processes a plurality of semiconductor wafers by a desired thermal process by placing a wafer boat, i.e., a workpiece holder, holding the semiconductor wafers in a horizontal position at predetermined vertical intervals, namely, pitches, in a reaction vessel, evacuating the reaction vessel to create an atmosphere of a negative pressure in the reaction vessel, supplying predetermined process gases into the reaction vessel, and heating the workpieces at a predetermined processing temperature by a heating means.
A wafer boat, such as having a holding capacity of 100 wafers, has, for example, 120 workpiece holding spaces. The wafer boat is fully loaded with semiconductor wafers by placing wafers to be subjected to a desired thermal process (hereinafter, referred to as “objective wafers”) in all the workpiece holding spaces in an objective wafer holding region, and placing a plurality of side wafers for stabilizing the thermal process in the rest of the workpiece holding spaces in side wafer holding regions respectively extending above and below the objective wafer holding region. The wafer boat thus fully loaded with the objective wafers and the side wafers is placed in the reaction vessel to subject the objective wafers to the thermal process.
Various types of semiconductor devices have been needed in recent years, and semiconductor wafers often need to be subjected to diverse thermal processes in small lots to manufacture semiconductor devices of diverse types. Therefore, wafer boats are often not fully loaded. In some cases, a wafer boat having a holding capacity of 100 objective wafers is loaded with, for example, fifty or twenty-five objective wafers for a thermal process.
In such a case, a number of dummy semiconductor wafers corresponding to the number of the workpiece holding spaces not occupied by the objective wafers are placed in the unoccupied workpiece holding spaces to load the wafer boat, and the objective wafers thus held on the wafer boat are subjected to a thermal process under the same processing conditions as those for processing objective wafers occupying all the workpiece holding spaces.
Usually, dummy semiconductor wafers that are placed in the unoccupied workpiece holding spaces in the objective wafer holding region are, for example, silicon wafers that are the same in quality as the objective wafers. The dummy wafers are cleaned every several processing cycles, and are thrown away after being used repeatedly. Thus, the dummy wafers increase the running cost and hence it is difficult to process objective wafers efficiently by a thermal process.